1. Field of the Invention
This invention relates to improved apparatus and method for measuring the thermal emissivities of surfaces and coatings.
2. Background of the Art
To simplify the following discussion the term heat loss and the term emissivity are used and the object or surface under discussion is considered to be at a higher temperature than its environment. However, it should be noted that an object can gain or lose heat by conduction and convection to a vapor or gas and can gain or lose heat by radiation. Therefore the principles that apply that allow an object or surface to be cooled or to dissipate heat when the environment is at a lower temperature than the object or its environment also apply to cause an object or surface to be warmed when the environment is at a higher temperature than the object or surface.
When warmer objects are placed in colder environments and must be kept warm, enough heat must be supplied to the object to offset any heat loss to the colder environment. In a gaseous environment, like that on earth's surface, heat can be lost from the object or gained by conduction and convection to the surrounding gas and by radiation. Heat loss by convection and convection can only occur at substantial rates in such a gaseous environment. Since in outer space there is not significant concentration of gases that could effect heat loss from the object, knowledge of the expected amount of heat loss from radiation would affect the amount of heat or energy that must be planned to be supplied to the object to keep it adequately warm. Radiative heat loss from an object occurs at the object's surface. The amount of heat leaving the object's surface depends on the temperature difference between the object and the place receiving the radiation and a surface characteristic. This surface characteristic is called Emissivity. Similarly, direct radiation from the sun could add to possible internal heat loads demanding cooling. The surface characteristic that affects the rate at which a surface absorbs energy radiated to it is called Absorptivity. In deep space the so-called heat sink temperature (the place receiving the heat) is in the range of 2.5 to 3.0 K on the absolute Kelvin scale.
Knowledge of the emissivity and absorptivity of the exposed surface of anything sent into space is an important requirement since these provide an accurate measure of the amount of heat that must be provided within to balance that lost by radiation. Large panels having high emissivity surfaces are used to dissipate heat that has been generated inside or absorbed by external surface of a space craft.
Emissivity is a measure of the rate at which a surface at one temperature radiates heat to another surface or body at a lower temperature. Absorptivity is like Emissivity except it is a measure of the rate at which a surface at one temperature absorbs heat from a surface or body at a higher temperature. Emissivity is expressed as a dimensionless number between 0 and 1 defined as the ratio of energy emitted from the object's surface to the energy emitted from the surface of a blackbody or perfect emitter at the same conditions. Spectral Emissivity is a special case directed to the emissivity of a surface at a given spectral color or wavelength.